Embodiments of the invention relate generally to true time delay (TTD) beam formers for an electrically steerable array antenna or phased array antenna, and more particularly to TTD beam former modules incorporating radio frequency (RF) micro-electromechanical systems (MEMS) switches.
Electronically steered antenna (ESA) systems or phased array antenna (PAA) systems combine the signals from multiple stationary antenna elements to point a beam of radio waves at a certain angle in space. The characteristics and angle of the beam is controlled in a manner that electronically steers the beam in different directions without physically moving the antennas. The electronic beam steering in a phased array antenna may be accomplished in one of two ways: through the use of phase shifters and by performing true time delay (TTD). TTD beam steering differs from a phase shifter type approach in the inherent bandwidth of the device and the fact that the device imparts a time delay rather than a phase shift. These distinctions allow the TTD device to be used in very wideband applications for forming antenna beams and nulls. This is advantageous for electronic warfare systems and broadband communication applications.
Beam steering via TTD is accomplished by changing the excitation time of each antenna element. A TTD module is fabricated with high speed switches coupled to transmission lines of various lengths. The amount of time it takes for a signal to be transmitted between the electronics and the antenna is controlled by selecting a particular combination of transmission lines, which imparts a desired amount of phase or time delay on the RF signal. Selection of the transmission lines may be accomplished using different types of switching elements, including utilize RF MEMS, which provide beneficial isolation and insertion loss properties that are advantageous for implementing in TTD applications. These RF MEMS switches use an electrically actuated mechanical movement to achieve an open circuit or a closed circuit in a RF transmission line. When the RF MEMS device is in an on-position, the RF transmission line is “closed” and the RF MEMS device can be used to transmit a high-frequency RF signal.
Despite the performance benefits resulting from the incorporating individual RF MEMS switches into ESA or PAA systems, the development of a small, low power, broadband antenna systems has proven to be a considerable challenge. Existing ESA and PAA systems utilize high power dissipation technology for the phase shifting, power combining, amplifiers, and signal conversion components provided within the system. Use of these components results in complex, costly, and heavy system architecture that has high thermal dissipation and is not suitable for highly miniaturized mobile communication applications.
Additionally, TTD modules may experience interference between the individual signal transmission lines, which degrades the beam steering performance of the TTD module. Such interference is especially prevalent in TTD modules utilized in broadband frequency signal processing applications as a result of the large number of transmission lines utilized to achieve the desired delay. An 8-bit TTD module, for example, requires 256 states, among which the long electrical length typically causes half wavelength (i.e., λ/2) resonances. These resonances occur at the selected signal line that is switched “ON” to accomplish the desired delay time and at the neighborhood transmission line that is switched off as a result of electromagnetic coupling. This coupling causes an undesirable “suckout” resonance in the selected states and results in poor signal transmission at particular beam states within the frequency band. While amplifiers can be added to improve signal performance, doing so adds undesirable size and cost to the phased array antenna system.
Therefore, it would be desirable to design a TTD beam former module with low RF insertion loss that effectively eliminates unwanted bandstop resonances and maintains good signal transmission for broadband frequency signal processing applications. It would also be desirable for such a TTD beam former to enable fabrication of a low cost, small-scale, and lightweight ESA or PAA system.